Predict your next period date, fertile window, and ovulation day for the next 6 months. Based on your last period date and average cycle length. Accurate menstrual cycle tracking for period prediction, family planning, and cycle awareness. Free, instant, private — no sign-up needed.
Next period, fertile window & ovulation — 6-month forecast
The average menstrual cycle is 28 days, but research from the Apple Women's Health Study (one of the largest US menstrual health studies, with data from over 60,000 participants) shows that only 13% of cycles are exactly 28 days. The normal range is 21–35 days with a period duration of 2–7 days. Cycle length can vary by up to 9 days from month to month even in otherwise healthy women — making accurate tracking important. Ovulation typically occurs 12–16 days before your next period (not 14 days after your last period as commonly stated) — this is why the "fertile window" shifts for women with longer or shorter cycles.
Period tracking provides valuable health insights beyond family planning. Irregular periods (outside the 21–35 day range or varying by more than 9 days) can indicate conditions like PCOS (polycystic ovary syndrome), thyroid dysfunction, perimenopause, or high stress levels — all of which warrant evaluation by a healthcare provider. The American College of Obstetricians and Gynecologists considers the menstrual cycle a "vital sign" — as important as blood pressure for evaluating reproductive and overall health. Apps and calculators like this one support cycle awareness but are not a substitute for contraception — typical use effectiveness of the fertility awareness method is 76–88%.
Menstrual phase (days 1–5): uterine lining sheds. Follicular phase (days 1–13): estrogen rises, follicles develop. Ovulation (day 14 avg): egg released from dominant follicle. Luteal phase (days 15–28): progesterone rises to prepare for potential implantation, then drops triggering next period if no pregnancy occurs.
Sperm survive 3–5 days in the female reproductive tract. An egg is viable for 12–24 hours after release. The fertile window is therefore 5–6 days before ovulation through the day of ovulation. This calculator estimates the fertile window as days 10–17 of a 28-day cycle (adjusted for your cycle length). For conception, timing intercourse in this window maximizes chances.
See your OB-GYN if: periods are consistently outside the 21–35 day range; you bleed for more than 7 days; your flow is extremely heavy (soaking a pad/tampon hourly for several hours); you experience severe cramping (dysmenorrhea); your period stops for 3+ months (amenorrhea) without pregnancy; or your cycle varies by more than 9 days month to month.
Consistent tracking over 3+ months provides the most accurate predictions. Log: start date, end date, flow level, and any symptoms (cramping, mood changes, spotting). After 3 months, your average cycle length becomes clear. Stress, illness, significant weight changes, and new medications can all temporarily disrupt cycle regularity.
Period prediction adds the average cycle length to the first day of the last period, and its accuracy depends entirely on how regular that length is. A normal adult cycle runs 21 to 35 days, and variation of a few days between cycles is entirely typical rather than a sign of anything wrong. Using an average of several recent cycles rather than the single most recent one improves prediction noticeably. Cycles are also legitimately irregular in the years after menarche and approaching menopause, when calendar prediction becomes unreliable regardless of method.
Next period = Day 1 of last period + average cycle lengthAverage cycle = sum of recent cycle lengths ÷ number of cyclesPredict n cycles ahead: Day 1 + (n × average cycle length)Variability = longest recent cycle − shortestwhere:
Assumptions: Calendar prediction only. Stress, illness, travel, weight change, exercise load and hormonal contraception all shift timing. Cycles consistently outside 21–35 days, or a sudden change in pattern, are worth discussing with a clinician.
Use several recent cycles rather than the last one, and quantify the uncertainty.
ResultNext period around 2 April — realistically 31 March to 4 April
Using only the most recent 32-day cycle would predict 4 April, two days later than the four-cycle average. Prediction windows widen the further ahead you project, which is why six-month forecasts in tracking apps should be treated as rough planning rather than dates.